Wireless navigation devices, e.g. optical mice, are becoming increasingly popular. A wireless optical mouse requires an internal power source because it is not connected to an external source of power. A wireless optical mouse typically includes an internal power source that supplies power to an optical navigation chip, a light source, a microcontroller and a transmitter. The most common internal power source is a battery, either rechargeable or disposable. The battery life is determined by the total power consumed during mouse operation. Increased battery life is an important goal, as the battery life of contemporary wireless optical mice is considered inadequate.
The light source in a conventional optical mouse accounts for approximately one quarter of the total power consumption of the mouse. The power required for the light source depends on the behavior of the photosensors of the optical navigation chip surface. To operate correctly, the photosensors of an optical navigation chip must receive a certain amount of light during the time that the photosensors are active. The photosensors will not provide useful information if either too little light is received (no signal) or too much light is received (saturation). Two basic approaches are used to assure that photosensors received a controlled amount of light while they are active. The first approach is to use a constant light source and to switch the photosensors from an active state to an inactive state when enough light is estimated to have been received during each sample period. The second approach, controlled light source technique, is to change the duty cycle and/or intensity of the light source to adjust the amount of light received by the photosensors during each sample period. If power savings is a concern, then the second approach is superior, because the light source is illuminated only during the periods that the photosensors require illumination.
When using the controlled light source method described above, power usage is minimized when the fraction of light reaching the photosensor array is maximized. For conventional optical mice, the navigation surface below the mouse is assumed to cause the light source to be scattered uniformly from the surface. For such a surface, light received by the photosensors is maximized when the photosensor array is parallel to the navigation surface and is centered over the illuminated portion of the navigation surface. As the photosensor array is moved away from being directly centered above the illuminated area of the navigation surface, the fraction of light received by the photosensor array is expected to decrease. However, most surfaces do not cause uniform scattering and many have a stronger component in the specular direction.